This invention relates to an improved method and apparatus for melting metal scrap such as aluminum. However, this disclosure is not limited to use with aluminum, but rather, is relevant to all molten metals.
Aluminum scrap can be divided into two general categories. The first category of scrap is comprised of large parts such as internal combustion engine parts which are generally self-submerging. The second category of scrap is termed lightweight scrap such as shredded food and beverage containers or machining chips and filings. The lightweight scrap is difficult to submerge and thus is difficult to melt.
Conventional melting systems for lightweight scrap have the problem that the floating scrap can build up in the melting bay, seriously interfering with the efficiency of the process. Such conventional systems also lead to higher levels of skim formation and melt loss resulting from the floating aluminum scrap that is converted to oxide and free aluminum metal that is occluded in the skim. In addition to the melt loss, high levels of skim require more intense processing downstream to separate these materials in order to provide purified metal.
U.S. Pat. No. 4,128,415 discloses a system for melting metal scrap in a molten media which includes a housing generally cylindrical in cross section and having upper and lower portions. Metal scrap is introduced to a body of molten melting media contained in the upper portion of the housing. A supply of molten melting media is added to the upper portion of the housing through a volute located in the lower portion. The molten melting media is supplied or added by action of an impeller located in the lower portion and mounted on a drive shaft extending through the upper portion. Vanes are mounted on the drive shaft to control the flow motion of the body of molten melting media and metal scrap in the upper portion of the housing by creating a vortex in this body for purposes of mixing the molten melting media and the metal scrap.
U.S. Pat. No. 3,997,336 discloses a system for melting metal scrap in a molten melting media which comprises a housing having an upper portion wherein the melting media and scrap are brought together to initiate melting. The housing also has a lower portion in which a volute is located. An impeller, having a central hub, a circumferential band surrounding the hub, and canted vanes projecting radially from the hub to the band, is positioned in the lower portion of the housing to cooperate with the volute so that upon rotation of the impeller, metal scrap and melting media are moved downwardly and out of the housing.
U.S. Pat. No. 4,518,424 discloses a method of melting metal scrap in a molten melting media. The method comprises the steps of providing a body of molten melting media in a housing having an upper portion and a lower portion, the lower portion having a generally cylindrical wall section. A supply of metal scrap is added to the housing and a supply of molten melting media is introduced to the upper portion of the housing. Melting of the metal scrap is initiated by ingesting it and directing the molten melting media downwardly in the housing by action of an impeller positioned in the lower portion, the impeller having a flat ring member having an opening in the center thereof and having blades extending from said ring member to a substantially circular disc member.
U.S. Pat. No. 4,486,228 discloses a method of melting metal scrap in a molten melting media. The method comprises the steps of providing a body of molten melting media in a housing having an upper portion and a lower portion, the lower portion having a generally cylindrical wall section. A supply of metal scrap is added to the housing and a supply of molten melting media is introduced to the upper portion of the housing. Melting of the metal scrap is initiated by ingesting it and molten melting media downwardly in the housing by action of an impeller positioned in the lower portion, the impeller having a flat ring member having an opening in the center thereof and having blades extending from said ring member to a substantially circular disc member. The scrap and melting media enters the opening in the ring member in an axial direction and is propelled therefrom in a radial direction by use of the blades. The impeller is positioned in the cylindrical wall section such that at least the ring member cooperates therewith to move the scrap and melting media from the upper portion through said impeller while substantially avoiding recirculation of the molten melting media within the housing to the upper portion.
U.S. Pat. No. 4,437,650 discloses an apparatus for melting relatively large floating units of metal scrap in a molten melting media or medium, the units having oxide films and solid, liquid and gaseous inclusions. After the units are charged into a melting medium, layers of freshly melted metal are presented to the molten media. The apparatus includes a bay for heating the molten media and means for pumping the media from the heating bay to a circular bay for receiving the large units of metal scrap.
U.S. Pat. No. 4,286,985, the disclosure of which is herein incorporated by reference, discloses a vortex melting system for ingesting and melting metal scrap that otherwise tends to float on the surface of a molten melting media. The method includes the steps of providing a supply of the melting media and directing the media from the supply to an upper portion of a receptacle having an outlet opening in the lower portion thereof. The flow of the melting media entering the receptacle produces a vortex of the media in the receptacle, as the media flows out the lower opening. The amount of the flow of the melting media to the receptacle and the size of the lower opening are such that a predetermined level of the media is maintained in the receptacle.
U.S. Pat. Nos. 6,036,745; 6,074,455; and 6,217,823 also describe metal scrap submergence devices. The disclosures of each of these patents is herein incorporated by reference.